Planar array antennas when imposed to cover multiple directions, suffer from scan loss. Since the projected aperture decreases as the beam is steered away from the broadside position which is normal to the ground surface and centered to the surface itself, it follows that broadside excitation of a planar array yields maximum aperture projection. Accordingly, when a beam from such an antenna is off the normal axis, the projected aperture area decreases causing a scan loss which is a function of cosine having a value of 1 with the argument of zero radians (normal) and having a value of 0 when the argument is .pi./2. ##EQU1##
There are a number of methods of beam steering using matrix type beam forming networks that can be made to adjust parameters as directed from a computer algorithm. This is the basis for adaptive arrays.
When a linear planar array is excited uniformly to produce a broadsided beam projection, the composite aperture distribution resembles a rectangular shape. When this shape is Fourier transformed in space, the resultant pattern is laden with high level side lobes relative to the main lobe. The ##EQU2## function is thus produced in the far-field pattern. In most practical applications these high level side lobes are an undesirable side effect.
Additionally, changes in the environment surrounding a communication array or changes at a neighboring communication array may require adjustment of the radiation pattern of a particular communication array. Specifically, seasonal changes around a base transceiver station (BTS) site can cause changes in propagation losses of the signal radiated from a BTS. For example, during fall and winter deciduous foliage loss can cause a decrease in signal path loss. This can result in unintentional interference into neighboring BTS operating areas as the radiation pattern of the affected BTS will effectively enlarge due to the reduced propagation losses.
Likewise, an anomaly affecting a neighboring BTS may cause an increase in signal path loss, or complete interruption in the signal, therefore necessitating the expansion of the radiation patterns associated with various neighboring BTSes in order to provide coverage in the affected areas.
Previously, crews have had to be dispatched to purposely tilt BTS antennas up or down to minimize interference or provide coverage in neighboring areas. Likewise, crews have again had to be dispatched when the anomaly affecting the signal has dissipated or been resolved. It becomes readily apparent that compensation for such anomalies, even occurring only seasonally, can be quite expensive. Furthermore, as the communication system grows in complexity, more such adjustments have to be made to bring the system back up to full operating capacity.
Accordingly, a need exists in the art for an antenna system which provides for azimuthal beam placement about an array to provide multi-directional coverage without using the aforementioned adaptive techniques.
A further need exists in the art for such an antenna system whereby the beam aperture is relatively constant and broadside to its intended direction without producing undesirable high level side lobes.
A still further need exists in the art for an antenna system which provides for automated control of the various beams comprising a radiation pattern.
These and other objects and desires are achieved by an antenna design which relies on a composite of antennas to provide multiple beams which are automatically, or remotely, adjustable.